1. Field of the Invention
The present invention is in the field of electronic component cooling, and in particular relates to the application of liquid spray phase-change cooling technology to remove excess heat from laser systems.
2. Description of the Prior Art
Most commercial laser systems use heat exchangers or chillers to remove excess heat. Usually, water or some other coolant flows across the heated regions, taking up heat, which is moved and dissipated in a heat exchanger or large tank. Some systems simply use a flow of city water through a laser head, then into a drain. Flow rates of greater than 10 gallons per minute are not uncommon. Liquid flow systems offer high heat removal capacities and are therefore very useful, if not essential, in lasers that generate a large amount of waste heat. Liquid coolers are usually quite large and use a significant amount of electrical power, and are therefore not easily portable. They also offer only a limited accessible temperature range and rather slow temporal response due to the physical properties of the fluid (viscosity, freezing points, etc.) and the typically large thermal masses of the reservoir.
Other laser systems use convective airflow over a heat sink (probably finned) attached to the excess heat source. This type of cooling is desirable and efficient when practical, since the power to drive a fan(s) may be all that is required. Because airflow cooling is typically less effective at removing heat than other methods, the technique works best in smaller lasers that do not dissipate much heat or require precise temperature control.
When better temperature control or low temperature operation is needed, thermoelectric (TEC) or other electrically driven coolers are often used to control the rate of heat removal, and therefore the temperature. However, the amount of power required to drive the electric cooler often exceeds that required to power the laser device, thereby lowering the overall system efficiency and increasing the demands on heat removal. Furthermore, electric chillers used to drive materials to low temperatures usually have limited capacity, and are therefore not usually used in higher heat load laser devices.
Another cooling method used in lasers is direct impingement cooling where the excess heat source in the laser is connected to a heat pipe or cold finger that is in thermal contact with a cold bath like liquid nitrogen. The heat flows into the cold bath and is eventually released as an increase in the vaporization rate. This can be a very efficient means of removing heat and cooling a laser device, with a similar heat removal capacity per volume as the liquid spray approach. However, the operating temperature is essentially fixed at the boiling point. Also, special cryogenic equipment and a supply of fluid are necessary for operation.
The waste heat removal system of U.S. Pat. No. 5,453,641 involves a coolant passing through a surface in thermal contact with a heated region via microchannels that induce capillary C action. The coolant vaporizes outside the microchannels for additional cooling. This system is effective but complex.
The proposed liquid spray phase-change cooling technology of the present invention provides an alternative way to remove excess heat from laser systems and to provide rapid cooling and stable temperature control over a wide range of temperatures. The invention could be particularly useful in laser applications where portability and electrical power consumption are at a premium, especially in applications where the laser run time is inherently limited. Examples could be man or vehicle carried laser systems for illumination, remote sensing, ranging, free-space communications, mobile satellite uplinks, or directed energy weapons. In addition, the technology could be used to efficiently cool laser materials (like laser diodes or three level solid state media) that perform better at low temperatures.
The surface to be cooled is connected to the excess heat source of a laser. A liquid coolant is sprayed onto the surface via a valve with rapid on/off response times. The valve is driven by a control signal in response to a temperature sensor. The surface temperature is thereby controlled by varying the spray duration and frequency. The coolant vaporizes at the surface and is either vented to the atmosphere or recirculated through a compressor and reliquefied.